In the cellular wireless communication system, with the increasing of the number of user, the jamming of the frequency spectrum and the distortion from the same channel become worse. To solve such problems, it is desired to improve the quality of cellular wireless communication system, to increase the coverage area of base station, and to increase the capacity of system by using smart antenna technology and array signal processing technology.
For a base station in the wireless communication network, there are signals coming from the mobile terminals in local cell as well as the signal with same frequency coming from the mobile terminals in other cell inside the received uplink signal. The more the cell of wireless communication network, the worse the same frequency distortion, thereby same frequency distortion will directly affect the receiving effect of base station and mobile terminals, inducing the reduce of base station's effective cover radius and degrading the quality of communication. To solve the problem of same frequency distortion, the base station need to selectively receive the uplink signal for depressing the distortion and to realize the directionally transmitting of the downlink signal, that is, it should direct the main lobe of downlink beam to the desired user, but form the zone point or side lobe of beam with lower power at the direction of other users. In this way, it can reduce the same channel distortion between cells, increase the coverage of cell and improve the quality of communication.
It can adopt the conventional beam forming technology to realize the directionally receiving and transmitting, such as adaptive beam forming technology. When adopting the adaptive beam forming technology, the main lobe of the beam can be directed to the desired user all along, and it can follow the movement of the user. In time division duplex (TDD) mode, the propagation paths of uplink and downlink signals are symmetry, that is, the propagation paths of uplink and downlink signals are same, but the propagation directions are opposite, so that the beam weight calculated through adaptive algorithm can be directly used for downlink direction, and realize directional transmission. But, there are altitude and phase errors between the receiving channel and transmission link, the directional accuracy will be affected by the directly usage of uplink beam weight, so it is need to calibrate the uplink, downlink channel and feeder cable to compensate the altitude and phase errors. In the frequency division duplex (FDD) mode, due to the frequency points of uplink and downlink are different, so that the channel of uplink and downlink are different, then the uplink weight can be directly applied for downlink. But the directions of arrival (DOA) can be estimated by the uplink, DOA information can be used to downlink directional transmission, but it is need to form the beam with given direction, as well as to calibrate the transmission link and feeder cable.
Beside adaptive beam forming method, the fixed multi-beam method which is relatively simply can be used to realize the directional receiving and transmission. The forming of fixed multi-beam is to use several fixed directional uplink beams and downlink beams to cover the receiving and transmission of the whole sector. The beam weight to form the fixed multi-beam can be determined by simulation method, but it can not determine the altitude and phase specialty of uplink and downlink channel in actual system during the simulation, so it can only assume the altitude and phase specialty of uplink and downlink channel are same, and it is similar to adaptive beam forming method, it must calibrate the transmission link and feeder cable to correctly form the downlink beam.
In wireless communication system, the common downlink channel is transmitted by broadcast. If it adopts the digital beam forming, a set of omni-beam weight can be gotten from simulation calculation and can be omni-broadcasted to cover the sector, otherwise, it can get a wide beam weight through calculation and can be broadcasted to cover the downlink sector. If Butler matrix is used to form the beam, it is needed to calculate or measure the equivalent beam weight in before, then to form the beam secondly in base band to solve the transmission problem of downlink common channel. In the same way, it must calibrate the transmission link and feeder cable to correctly form the wide beam which is formed using the above method.
There are many methods about the calibrating of array antenna and transmission link, hereinafter are several conventional method:
(1) In U.S. Pat. No. 4,448,155 for “Method and apparatus for self-calibration and phasing of array antenna” and China Patent No. 01800020.7 for “Apparatus for array antenna wireless communication and method for calibration”, a kind of methods for calibrating the array antenna and transmission link is set forth, this method is to measure the gain and phase of each transmission link using instrument at the project field, then to calibrate the array using the measured result. The most disadvantage of this method is that it uses the off-line calibrating manner, and it can not calibrate the time-varied array error. It is not benefit to the engineering usage and maintenance, particularly, it is not suitable for the running communication system.
(2) In U.S. Pat. No. 6,615,024 for “Method and apparatus for determining signatures for calibrating a communication station having an antenna array”, China Patent No. 00815528.3 for “Method and apparatus for calibrating smart antenna array” and China Patent No. 02142694.5 for “Method and apparatus for calibrating antenna array”, a kind of methods to calibrate the array antenna is set forth. In these technical schemes, a beacon antenna and corresponding transceiver of calibrating signal need to be placed in the far field area and near field area of antenna array. During the calibration of uplink, the calibration signal is transmitted by beacon antenna and it is received by base station; during the calibration of downlink, the calibration signal is transmitted by base station and it is received by beacon signal, then the calibration weight is calculated out for the uplink and downlink of base station. The advantage of this calibration method is that the uniformities of the altitude and phase of the whole transmitting and receiving link can be calibrated, but due to the need of equipments such as transceiver of calibrating signal, beacon antenna, etc., so it increases the complexity and cost to realize the base station. Further, more significant multi-path effect will be induced when beacon antenna is placed in the far field area, and the calibrating signal is not planar wave when reaching antenna array in the case that beacon antenna is placed in the near field area, so it causes the calculation of calibrating weight is very complicated.
(3) In U.S. Pat. No. 6,600,445 for “Method and device for calibrating smart antenna array”, U.S. Pat. No. 6,236,839 for “Method and apparatus for calibrating a smart antenna array”, U.S. Pat. No. 6,157,340 for “Adaptive antenna array subsystem calibration”, China Patent No. 01112987.5 for “Method and apparatus for calibrating smart antenna channel array”, China Patent No. 01122536.X for “A close-loop calibrated dipolar smart antenna array system”, China Patent No. 01809947.5 for “Calibrating system of array antenna receiving apparatus”, and China Patent No. 03102791.1 for “Apparatus for calibrating array antenna and method for calibrating array antenna”, a kind of methods for calibrating array antenna is set forth in each of them. In this kind of methods, it is considered that the non-uniform error of the gain and phase for each antenna unit and corresponding antenna feeder cable is not time-varied, firstly, the gain and phase for this part is measured using instrument and the measured results are saved, then a coupler and the calibration transceiver coupled to this coupler are designed for the RF (radio frequency) terminal of each link, the calibration method is similar to that of (2), due to the calibration weight got from calculation is not the calibration weight for the whole link, so it need to be modified by the measured result. Compared with the methods in (2), the advantage of this kind of method is not using beacon antenna and corresponding antenna feeder cable, but it still need the transceiver of calibrating signal, it need to measure the altitude and phase specialty of antenna unit and antenna feeder cable, it causes the complicated calibration equipment and high cost.